An apparatus for irradiating substances curable by radiation of a selected spectral range is known from German Offenlegungsschrift No. 26 07 249. A lamp commonly used for movie projectors and including an aluminum reflector serves as the light source, and the entire radiation of the lamp which includes visible, ultraviolet and infrared spectral ranges, is focussed by the reflector to a focus situated in front of the lamp. A filter is disposed within the radiation cone between the reflector and the focus, which reflects the UVA spectral portion (320 to 400 nm) and transmits the remaining radiation. The filter is a thin filter layer deposited by evaporation onto a support plate of quartz glass extending at 45.degree. with respect to the optical axis of the radiation cone. The portion of the radiation reflected by this filter is coupled into an optical wave guide and thereby directed onto the object to be irradiated.
The known apparatus is intended primarily for curing dental fillings of synthetic material polymerizable by ultraviolet radiation within the respective dental cavity. In this application, only the UVA-radiation is applied to the filling while both the longer-wave visible and thermal radiation and the unholesome shorter-wave radiation is filtered away. This undesired radiation transmits the filter and impinges on a thermal shield which is cooled by ventilation.
In the known apparatus, the deflection of the useful radiation from the optical axis of the radiation cone produced by the lamp is desired as it ensures that, even when this filter is damaged, no radiation of the undesired spectral ranges will be transmitted through the optical wave guide and thus to the object to be irradiated.
The known irradiation apparatus, however, has a number of severe disadvantages. The filter is exposed to a very high thermal load because it reflects only a small portion of the entire radiant power and because a portion of the remaining transmitted power is absorbed by the supporting plate of the filter. Furthermore, since the lamp is operated intermittently, the thermal load occurs as a changing load causing particular stress on the filter. Thus, the plane and thin filter glass plate, being in itself very fragile, forms a truly weak point of the known apparatus.
In order to make the apparatus as small and handy as possible and also to make the delicate filter plate as small as possible while simultaneously utilizing the full light cone produced by the lamp and the reflector, it is furthermore required to grind the filter at least partially oval, which causes difficulties in the manufacture due to the fragibility and is accordingly expensive.
For reasons resulting from the use of a commercially available light source having a standard reflector shape and again from the desire to make the entire apparatus as small and handy as possible, the space between the radiation cone produced by the light source and the input end of the optical wave guide is very limited so that it is difficult to mount additional filters between the first-mentioned filter and the optical wave guide without interfering with the light cone. Such additional filters may be necessary depending on the frequency characteristic of the thin-film filter.
A further and possibly the greatest problem of the known apparatus results from the fact that the filter characteristic of the thin-film filter used depends on the angle of incidence of the light beam. In case the thin-film filter is selected such that maximum reflection occurs at the average angle of incidence of 45.degree. at a wave length of e.g. 350 nm, this maximum is moved towards smaller wave lengths with increasing angle of incidence and to greater wave lengths with smaller angles of incidence, as shown in the diagram of FIG. 3. Such greater and smaller angles of incidence, however, cannot be avoided with the geometry of the known apparatus. Indeed, a deviation of .+-.30.degree. from the average angle of incidence of 45.degree. must be taken into account. This causes a substantial shift of the filter characteristic for the peripheral portion of the radiation cone, resulting in less reflection of the desired spectral range and more reflection of the undesired and partly even harmful spectral range which latter must be absorbed by additional filters. It is furthermore to be considered that the near-axis rays which would be incident at the optimum angle of 45.degree. occur not at all because of the evacuation nipple produced during manufacture at the tip of the lamp bulb and because of the absence of the central reflector portion where the lamp itself is mounted.
Moreover, in consideration of unavoidable tolerances in the mounting to the filter within the housing at the 45.degree. position, a further deterioration of the filter function must be expected. Also, the mounting must take into account the above-mentioned thermal loads of the filter by allowing for an according play.
As a further disadvantage, the filter, the reflector and the input end of the optical wave guide together form a substantially closed chamber in which the heat produced by the light source will accumulate and put additional load on the reflection filter as well as any other absorption filters provided at the input of the optical wave guide.
In the field of movie and slide projectors, German Offenlegungschrift No. 1 572 759 and Austrian Pat. No. 215 180 disclose a reflector which substantially reflects only visible light and transmits thermal radiation. However, if such a reflector were used in an irradiation apparatus operating in the UVA range or near-UVA visible range for dental applications, at least one additional filter would be required to be disposed in the reflected radiation in order to separate this desired spectral range from the remaining, visible light. Using a reflection filter as such additional filter would result again in most of the above-discussed disadvantages.* If an absorption filter were used as the additional filter, there would be the further disadvantage that this filter is exposed to a particularly high thermal load and the risk of fracture resulting therefrom. Failure of this filter would entail the further disadvantage that the visible light would not be prevented from being transmitted by the optical wave guide to the object to be irradiated, where it will regularly interfere. FNT *particularly since a deflexion from the optical axis would be unavoidable.
It is an object of the invention to provide an apparatus for irradiating substances curable by radiation of a selected spectral range, which avoids at least part of the above-mentioned deficiencies and which, particularly, allows a more exact filtering, thus a better exploitation of the desired spectral range in the radiation produced by the light source. It is a further object to produce an apparatus of this type in which the thermal load on the various elements of the apparatus is reduced.
It is another object of the invention to provide an apparatus of the mentioned type with a structure which is inexpensive to manufacture and easy to handle.
As a further object of this invention, an irradition apparatus of the described type is to be achieved which provides an unambiguous warning to the user in case of malfunction of the radiation source.